This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-021335, filed Jan. 29, 1999; the entire contents of which are incorporated herein by reference.
The present invention relates to a laser radiating apparatus for radiating a laser beam to an object, and methods for manufacturing a polycrystalline semiconductor film, for example, non-monocrystalline semiconductor film and a liquid crystal display device using the laser irradiating apparatus.
When a semiconductor device or a liquid crystal display device is manufactured, a photolithography step and an anneal step require radiation of light. In these steps, a thin film is processed by a laser beam.
For example, in the photolithography step, a mask image is transferred to photoresist on the semiconductor substrate by radiation of a laser beam emitted from a laser oscillator. In the anneal step, an amorphous semiconductor film, i.e., a non-monocrystalline semiconductor film, is annealed by radiation of a laser beam emitted from an excimer laser oscillator (ELA: Excimer Laser Anneal), so that it is polycrystallized. The process time is shortened by the excimer laser anneal process, resulting in improvement of the manufacturing efficiency (throughput).
The laser radiating apparatus for performing the aforementioned processes by means of the laser beam has a laser oscillator. The laser beam output from the laser oscillator is passed through an optical system comprising an optical element, such as a homogenizer. The optical system substantially homogenizes the distribution of intensity of the laser beam, and shapes the laser beam to a predetermined form of the beam radiation surface. Then, the laser beam is guided to and irradiates an object to be processed.
In a process using the laser beam as described above, the intensity of energy of the laser beam radiated on the object must be controlled with high accuracy to increase the accuracy of the process. To control the intensity of energy of a laser beam, part of the laser beam output from the laser oscillator is split by split means, for example, a semi-transparent mirror, and the intensity of energy of the split laser beam is detected. Based on the detection value, the intensity of energy of the laser beam output from the laser oscillator is controlled.
The laser beam for irradiating the object passes through an optical system comprising a plurality of optical elements, such as a homogenizer and a focusing lens. Since these optical elements or the like absorb the optical energy of the laser beam, an optical loss is caused in the light path in the optical system before the light beam reaches the object.
On the other hand, the laser beam split to detect the intensity of energy is directly incident on the detector without passing through an optical element. Therefore, there is substantially no loss in the optical energy of the intensity of energy detected by the detector.
For this reason, the intensity of energy of the laser beam detected by the detector is different from that of the light beam irradiating the object. In this state, if the intensity of energy of the laser beam output from the laser oscillator is controlled on the basis of the intensity detected by the detector, the intensity of energy on the radiation surface of the laser beam cannot be controlled with high accuracy. The control method is disadvantageous in this respect.
The aforementioned anneal process will now be described. In this process, an amorphous silicon film, i.e., a semiconductor film, formed as a thin film on the glass substrate is radiated with a line beam of a laser beam in the ultraviolet wave range, such as an excimer laser, as shown in FIG. 16. As a result, the amorphous silicon film can be polycrystallized to form a semiconductor film with high electron mobility.
In this process, since the amorphous silicon is instantaneously melted and crystallized, a polycrystalline silicon film can be formed by a low-temperature process at about 450xc2x0 C. or lower, in which the substrate is damaged little by heat. Thus, the process is advantageous in that a non-monocrystalline silicon film can be formed on a glass substrate, which is less heat-resistant, large in area, and inexpensive.
The electron mobility is represented by the equation: xcexc=|vd/E | (cm2/Sxc2x7V). It stands for an average drift speed (vd (cm/s)) of electrons in a crystal per unit field, when an electric field E (V/cm) is applied to the crystal.
The non-monocrystalline silicon also includes a state during a phase transition from amorphous silicon to polycrystalline silicon. The amorphous silicon to be annealed by the laser beam has a high purity, but the percentage of amorphous silicon is not necessarily 100. Therefore, when the amorphous silicon is annealed by the laser beam in the process described above, once the ratio of the polycrystalline silicon is increased while the amorphous silicon is decreased, the amorphous silicon can be used as non-monocrystalline silicon.
Use of the non-monocrystalline silicon film provides a thin film transistor (TFT) having high electron mobility on the glass substrate in the low-temperature process described above. With the non-monocrystalline silicon TFT, it is possible to obtain a thin and high-definition liquid crystal display device, called a driver monolithic type, which uses an active matrix substrate made of a driver TFT (complementary transistor) and a pixel TFT formed on a glass substrate.
In an LCD unit formed on a glass substrate, as shown in FIG. 17, the driver TFT is controlled by gate lines (scanning lines) and data lines (signal lines), so that an image is displayed by applying a voltage to the liquid crystal.
The driver TFT comprising, a gate driver and a source driver, controls the gate lines and the data lines. Each driver receives an image signal and a synchronous signal through a signal control unit and power from a power source unit. The gate driver is a digital circuit having a function of selecting every gate line in one frame (60 Hz), and operated at cycles of a scanning time (15 to 40 microseconds).
The source driver applies a voltage of the gate line to a liquid crystal filed between a pixel electrode made of a transparent ITO (Indium Tin Oxide) film on the array substrate and a counter pixel electrode on the counter substrate, so that a voltage in accordance with image information can be applied via the pixel TFT to the liquid crystal. Since the display in liquid crystal is deteriorated when a DC voltage is continuously applied thereto, voltages of the opposite polarities are alternately applied to the pixel electrode and the counter electrode (inversion driving). The source driver is driven at a high frequency of 20 to 100 MHz. Therefore, since the driver TFT is required to be operated at a high frequency, the electron mobility therein must be high.
To obtain a non-monocrystalline silicon having the satisfactory characteristics as described above, it is necessary to increase the accuracy in the anneal process by accurately controlling the intensity of energy of the radiated laser beam. However, according to the conventional art, since a laser beam having suitable energy intensity cannot be radiated on the object for the reasons described above, an anneal process with high accuracy cannot be performed.
An object of the present invention is to provide a laser radiating apparatus in which a difference in intensity of energy between a laser beam for detecting the intensity of energy and a laser beam radiated on an object is suppressed to the minimum, so that the intensity of energy of the laser beam radiated on the object can be set with high accuracy to carry out a satisfactory process.
According to the present invention, there is provided a laser radiating apparatus for controlling an intensity of energy of a laser beam output from a laser generating means and radiating an object with the laser beam, the apparatus comprising: intensity homogenizing means for substantially homogenizing distribution of the intensity of energy of the laser beam; converging optical means for condensing the laser beam substantially homogenized by the intensity homogenizing means and radiating the object with the laser beam; shaping means for removing a part of the condensed laser beam and shaping it into a predetermined beam shape; detecting means for detecting an intensity of energy of the part of the laser beam removed by the shaping means; and control means for controlling the intensity of energy of the laser beam based on a detection signal output from the detecting means.
With the present invention, when the intensity of energy of a laser beam to be radiated on the object is detected and controlled, a difference in intensity of energy between the laser beam for detecting the intensity of energy and the laser beam radiated on the object is suppressed to the minimum. For this reason, the intensity of energy to be radiated on the object can be controlled with high accuracy. In addition, since the intensity of energy of the laser beam is detected by means of an unnecessary laser beam removed by the shaping means, the intensity of energy of the laser beam for use in processing the object can be detected without loss.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.